1. Field of the Invention
The present invention relates to an image processing apparatus which encodes image information and stores encoded image data in a storage medium such as an optical disk, or decodes encoded image data read out from the storage medium and outputs the decoded image data to a display device, a printing device, or the like.
2. Description of the Related Art
In an image processing apparatus such as an image filing system, a facsimile apparatus, or the like, high-efficiency encoding processing is performed to efficiently store and transmit image data. As for high-efficiency encoding processing, various methods are available, and these methods can efficiently compress document image data constituted by character or line images. This is due to high redundancy of document image data.
A gray scale image of an original such as a picture is binarized by quasi-gray scale processing. However, image data obtained by this processing has low redundancy. This is because binary data corresponding to a region having a uniform gray scale intensity value in an original is intentionally set to have a change in black-and-white gradation. Such gray scale image data cannot provide a high data compression effect like document image data. On the contrary, encoding for achieving data compression may result in an increase in data volume.
As a typical binarizing method based on quasi-gray scale processing, a dot-concentrated ordered dither method (to be referred to as a concentrated dither method hereinafter) and an error diffusion method are conventionally known. In the concentrated dither method, black pixels are concentrated in proportion to the intensity level, and gradation is expressed by the size of a group of black pixels. For this reason, the resolution is lowered in the concentrated dither method. In contrast to this, in the error diffusion method, simple binarizing errors of a pixel of interest are distributed to neighboring pixels to scatter black pixels as much as possible, and gradation is expressed by a local density of the black pixels.
when an image scanner binarizes a gray scale image of an original by an error diffusion method which can maintain gradation and resolution, no problem is posed when image data obtained from the image scanner is stored in a storage medium such as the optical disk. However, when image data read out from the storage medium is output to a reproduction output device such as a CRT which has a lower resolution than that of the image scanner as an input device, a problem associated with reproducibility of gradation is posed. More specifically, a region where black-and-white gradation must finely change may be displayed entirely in black.
In order to prevent deterioration of image quality such as generation of moire in a concentrated dither image upon resolution conversion when image reproduction is performed by an output device having a resolution different from that of an input device, as described above, a method of performing resolution variation after an image is temporarily converted into a gray-scale image, and re-binarizing the gray-scale image is conventionally known in Japanese Patent Application (Kokai) Publication No. 4-205675, for example. However, since a quasi-gray scale processing method upon re-binarizing cannot be adaptively selected, a concentrated dither image is undesirably displayed in a region where an error diffusion image is to be reproduced and displayed.
As described above, a gray level image of an original such as a picture is binarized by quasi-gray scale processing. However, image data obtained by such processing has low redundancy, and coding for achieving data compression may result in an increase in data volume. In addition, when an image is output to a reproduction output device having a resolution lower than that of an input device, a region where black-and-white gradation must finely change may be displayed entirely in black, resulting in poor reproducibility of gradation.